Tracking the location of a transmitting wireless device, such as a cellular telephone, often involves the use of direction finding equipment. Conventional direction finding equipment can utilize a directional antenna to determine a direction to a transmitter. A direction finder typically includes several antennas that are used to find the location of a remote transmitter by analyzing the different arrival times of the RF signal from the transmitter that is received at the separate antennas. Because such systems do not generally provide range information, triangulation of the transmitter using multiple direction finding devices is required to fix a location of the transmitter. This can be difficult or impossible when the transmitter is being carried by a person who is walking, travelling in a car, or otherwise is moving. Moreover, when the transmitter is moving through an urban or other environment with structures or other features, multipath and scattering effects can make locating the transmitter that much more difficult. This is because the antennas may receive delayed versions of the RF signal, causing ambiguity and an incorrect location determination. Accordingly, in such environments, tracking a moving transmitter often requires the use of a tracking system that is itself mobile, so that a more or less direct line of sight to the transmitter can be maintained by following the transmitter with a direction finding device.
As an alternative to terrestrial tracking devices, it is possible to locate a transmitter using satellite or airborne detectors. In such systems, frequency difference of arrival (FDOA) and time difference of arrival (TDOA) techniques can be used. These systems are commonly used in proprietary systems, intelligence applications, and for monitoring of RF ground resources. However, such techniques are only effective when the transmitter has a clear line of sight to the transmitter and when the signals do not encounter multipath, fading, or any other RF selective path impairment. Alternatively, the channel characteristics of the environment, such as the multipath model, channel fading, and the like, must be well known. However, even when a particular environment is well characterized, the added Doppler and multipath effects when the transmitter is moving makes accurately locating the transmitter especially difficult. Accordingly, systems employing such techniques do not themselves provide a solution to the problem of tracking a moving transmitter in a clustered, urban or other environment in which direct lines of sight to the transmitter are not available, and in which multipath or scattering effects are present.